The synthesis of diamond materials is inherently expensive, and the nature of the typical deposition technologies involved make it difficult to coat a given surface with diamond in a manner that is economically attractive while also maintaining high functionality. Also, there are many examples in which functionality can be achieved but the durability of the coating is so poor as to again undermine the commercial viability of the technology:
Wear resistant coatings—The diamond must survive high shear stresses without delaminating from the substrate, yet be both smooth (low friction) and hard (wear resistant) across a broad spectrum of harsh conditions (face pressures, media containing abrasive substances/chemicals/particles, etc.)
Electrochemical electrodes—In order to reduce cost and to improve compatibility with other materials, diamond coatings must typically be integrated with conducting metal substrates with much higher thermal expansion coefficients as compared to diamond, and also with characteristics generally unfavorable for the nucleation and adhesion of diamond to the substrates (i.e. not good carbide-forming materials). Also the films must be pin-hole free to avoid electrochemical reactions to occur at the diamond/substrate interface which also can lead to delamination of the film from the substrate. The diamond must be doped with boron or other dopant to make it conductive, and the surface of the diamond that will drive most of the electrochemical reactions, (for which diamond is attractive) must consist of a large sp3-bonded fraction. The surface of the diamond must not consist of large amounts of sp2-bonded (graphitically bonded) carbon that can reduce the over-potential for oxygen evolution in water-based reactions, or reduce the chemical inertness of the film that is important in all electrochemical applications, including those that occur in aqueous environments.
Biomedical Devices—Commonly used biomedical materials such as Ti-alloys, high-density graphite, and ferrous alloys present unique challenges for the initial nucleation of diamond in order to produce films that will survive for long periods of time, in the body, without the need for replacement. Again, the chemistry that optimizes the properties of the interface between the diamond and the substrate are usually not those that optimize the surface of the diamond for the applications. For hip/knee replacements the diamond must be bio-inert, bio-stable, and resistant to wear and fouling, as well as being smooth and free from surface features that would lead to excessive wear of the counterface material (such as, Ultra High Molecular Weight PolyEthylene—UHMWPE or Cobalt-Chrome alloys). Heart valves consisting of pyrolytic carbon coated with diamond also must be optimized in a similar manner, with the use of pre-processing and initial growth chemistries that promote enhanced chemical and mechanical adhesion to the substrate, but are then changed during the deposition process so that the terminal surface of the diamond coating is optimized to be anti-thrombogenic.
Biosensors—again the adhesion to the substrate is important, but the terminal surface must promote the attachment of targeted biomolecules typically used to impart bioselectivity to the surface. An example is the covalent attachment of antibodies specific to E coli. H157 for the detection of this weaponized pathogen.
MEMS devices & Diamond-based micro-machines—For diamond thin films to be optimized for these applications, the films must be deposited on substrates than can be etched away to fabricate suspended structures of diamond (micro-cantilevers, comb-structures, etc.), yet the overall film must have a net zero residual film stress. These films may also need to be post-polished to deliver near atomic smoothness so that subsequence layers of other thin film materials can be deposited onto them. Extreme adhesion is required between the diamond layer and the substrate to prevent delamination during polishing, other processing and during operation of such MEMS devices.
A common element for the development of thin diamond coatings that are technically and commercially successful is the need to develop a series of process steps that optimize two critical attributes: the diamond/substrate interface—achieved through a combination of choice of substrate, pre-processing of the substrate (e.g. by roughening the surface and “seeding” it with diamond particles of a particular size), and a choice of initial diamond growth chemistry to maximize the chemical and mechanical bonding of the diamond to the substrate and also the uniformity of the growth across the substrate, which can be relatively large in dimension in comparison to the thickness of the film.
The terminal diamond surface achieved by a combination of growth chemistry (different from that of the chemistry used during the initial nucleation and growth) and post-processing of the film (i.e. lapping, polishing, chemical functionalization to terminate the surface with chemical species to further optimize its functionality for different applications).
Central to the innovation described here is the concept of functional, commercially viable synthetic diamond films that optimize both the diamond/substrate interface and the terminal diamond surface to enhance their attractiveness for actual applications of interest. The use of chemical vapor deposition (CVD) tools such as hot-filament CVD, microwave plasma CVD, and other CVD tools, allows for the chemistry of the diamond film being deposited to be adjusted during the growth so as to overcome the issues described above. Pre- and post-processing must also be used in order to accomplish the objective.
Diamond is well known to be a hard material by those unskilled in the art. For those skilled in the art it is generally well known that the properties of diamond thin films grown using conventional chemical vapor deposition technologies can be adjusted and optimized for different applications. Choices of deposition chemistries can, for instance, dramatically change the optical transparency or thermal conductivity of the material. In most cases engineering of the film for a particular property, results in the diminishment of other important film properties. High thermal conductivity requires growth chemistries that yield larger diamond grains, which have an overall negative impact on the differential stress of the film and the cost as well, i.e. slower deposition rates. Fine grain diamond materials that are well suited to achieve superior film adhesion and lower film stress yield less favorable thermal conductivities and are also not as optically transparent. Films grown to be highly thermally conductive are less smooth and are less desirable for applications that require low friction and high wear resistance. Therefore, there is a need to develop a novel diamond film which can simultaneously deliver several of the required properties without an increase in deposition cost.